An ink-jet printer is used in various kinds of image forming apparatuses (or image recording apparatuses) such as printers, facsimile machines and copying machines. The ink-jet printer can record at a high speed, and does not require a special fixing process with respect to the so-called plain paper. In addition, noise generated by the ink-jet printer during the recording is negligibly small. For this reason, the ink-jet printer is popularly employed for office use.
In the ink-jet printer, a pressure generating means of an ink-jet head is formed by a heating resistor which generates air bubbles in the case of the thermal ink-jet, a piezoelectric element in the case of the piezoelectric ink-jet, and an electrostatic element in the case of the electrostatic ink-jet. The dot size is controlled by controlling an amplitude, a pulse width, a number of pulses and the like of a driving voltage which is applied to electrodes of such pressure generating means. Hence, the pressure generating means applies pressure to an ink chamber of the ink-jet head based on image information to eject ink drops from a nozzle of the ink-jet head, and the ejected ink drops are adhered onto a recording medium such as paper and film so as to form an image of the image information on the recording medium.
The ink-jet printer can be categorized into a serial type and a line type depending on the head structure.
The serial type ink-jet printer forms the image while scanning the recording medium in a main scanning direction by the ink-jet head, that is, in a direction taken along a width of the recording medium (hereinafter simply referred to as a width direction). After one or a plurality of scans in the main scanning direction ends, the recording medium is transported to form a next recording line.
On the other hand, the line type ink-jet printer uses an ink-jet head having nozzles arranged along approximately the entire width of the recording medium. Hence, no scan is made in the width direction of the recording medium, and the recording is made while transporting the recording medium.
The line type ink-jet printer has an advantage in that the recording speed is high, because one fine in the width direction of the recording medium is formed simultaneously. However, the size of the ink-jet printer as a whole becomes large because the ink-jet head of the line type ink-jet printer is large. Furthermore, in order to carry out the recording at a high resolution, the nozzles must be arranged at a high density, thereby increasing the production cost of the ink-jet head.
The serial type ink-jet printer has an advantage in that the ink-jet head is relatively small, and the cost of the ink-jet printer as a whole can be suppressed, because the production cost of the ink-jet head of the serial type ink-jet printer is relatively low. For this reason, the serial type ink-jet printer is popularly used in various applications.
However, particularly when the ink-jet printer records the image on plain paper, picture quality deteriorations peculiar to the ink-jet recording, such as color reproducibility, durability, light resistance, ink drying characteristic (fixing characteristic), feathering, color bleeding and duplex recording characteristic of the image, become conspicuous. Moreover, when an attempt is made to carry out a high-speed recording with respect to the plain paper, it is extremely difficult to carry out the recording while satisfying all of these characteristics which affect the picture quality.
Next, a description will be given of the problems peculiar to the ink-jet printer when recording the image on the plain paper.
Normally, the ink used for the ink-jet recording (printing) includes water as a main component, and is generally added with a coloring agent, and a wetting agent such as glycerol for the purpose of preventing clogging of the nozzles. The coloring agent-includes a dye and a pigment, and conventionally, a dye-based ink is used for a color portion in most cases because of the superior color generation and stability.
However, the ruggedized characteristic, such as the light resistance and water resistance of the image, obtained by use of the dye-based ink is poorer than that obtained by use of an ink which uses the pigment as the coloring agent. With respect to the water resistance in particular, it is possible to realize an improvement to a certain extent by use of recording paper which is made exclusively for the ink-jet recording and includes an ink absorbing layer. But a satisfactory water resistance cannot be obtained when the plain paper is used.
In order to solve the problems cased by the use of the dye-based ink for the recording with respect to the plain paper, a pigment-based ink using an organic pigment, carbon black or the like as the coloring agent is used for the recording with respect to the plain paper. Unlike the dye, the pigment has no solubility to water. Hence, the pigment is normally mixed with a dispersing agent and subjected to a dispersion process to form a water ink in which the pigment is stably dispersed in water.
The light resistance and the water resistance can be improved by use of the pigment-based ink described above. However, it is difficult to simultaneously improve the other characteristics which affect the picture quality. It is difficult to obtain a high image tone, a sufficient color generation, a high color reproducibility and the like, particularly when carrying out a high-speed recording with respect to the plain paper. As a result, it is difficult to simultaneously obtain satisfactory ink drying characteristic (fixing characteristic), feathering, color bleeding and duplex recording characteristic of the image.
Recording methods have been proposed in Japanese Laid-Open Patent Applications No.6-171072 and No.2000-355159 in order to solve the problems introduced when carrying out the recording on the plain paper using the pigment-based ink described above.
According to the recording method proposed in the Japanese Laid-Open Patent Application No.6-171072, the ink includes a pigment, a polymer dispersing agent and a resin emulsion. By adjusting a solid adhering amount per unit area on the recording paper during a 100% duty recording to an appropriate range, it is possible to reduce the printing inconsistency caused by pigment agglutination peculiar to the pigment0based ink. As a result, the feathering is eliminated, and a picture quality having a high recording tone can be obtained.
On the other hand, according to the Japanese Laid-Open Patent Application No.2000-355159, the ink composition includes a pigment and a penetrant, where the pigment has a dispersion group at the pigment surface and is surface-treated so as to be independently dispersible into a water solvent. By adjusting an ejecting amount of the ink composition per unit area of the recording paper, it is possible to suppress the generation of irregular feathering in the recorded image, and to quickly dry the ink composition on the recording paper. As a result, a high recording tone can be secured, and a satisfactory picture quality can be obtained.
Next, a description will be given of the recording speed (printing speed). In the case of the serial type ink-jet printer, the recording speed is determined by a resolution of the image, nozzle density, driving frequency at which the dots are formed, sub scanning speed and the like.
The nozzle density is limited by the processing accuracy of the nozzle, ink chamber, flow passage, actuator and the like. Particularly in the case of the ink-jet head which uses the piezoelectric element as the pressure generating means, it is necessary to carry out a mechanical processing such as dicing or, to form a PZT thin film by printing, in order to divide and form channels corresponding to the nozzles. For this reason, the nozzle density becomes low compared to the thermal ink-jet head which is formed by a semiconductor process. Presently, the upper limit of the nozzle density of the ink-jet head using the piezoelectric element is approximately 360 dpi.
In addition, in order to improve the recording speed, it is desirable to record a recording region in one main scan. For example, when forming an image having a resolution of 300 dpi in the sub scanning direction using a head having a nozzle density of 300 dpi, the image can be formed in one scan in a moving direction (main scanning direction) of the head. But when forming an image having a resolution of 600 dpi in the sub scanning direction using the same head, the image must be formed by a so-called interlacing in which two scans are made in the main scanning direction and one scan is made in the sub scanning direction (paper transport direction). Naturally, the method which forms the image in one scan in the main scanning direction by a non-interlacing has a higher recording speed that the method which forms the image by the interlacing. Moreover, as methods of forming one line in the main scanning direction, there is a single-path recording (printing) method which forms one line by one main scan, and a multi-path recording (printing) method which forms one line by a plurality of main scans. Of course, the recording speed of the single-path recording method is higher than that of the multi-path recording method.
However, particularly in the case of the piezoelectric ink-jet printer using the piezoelectric element, the nozzle density itself is low, and for this reason, the resolution of the image inevitably becomes low when the single-path interlacing is employed to improve the recording speed.
When the resolution of the image is low, the picture quality can be improved by representing each pixel in multi-levels. The multi-level representation method may vary the size of each dot, form each pixel by forming a plurality of small dots or, varying the density of the ink itself.
The multi-level representation method is effective in the case of images such as photographs, but are virtually ineffective in the case of graphics and characters. In the case of graphics and characters, the dot size needs to be large so as to be able to fill the texture. For this reason, the tone of the graphics and characters becomes low when the dot size used is small. Accordingly, problems peculiar to the low resolution occur in the case of a bi-level image such as graphics and characters, and particularly in the case of the characters, the quality of the characters becomes poor and the characters become difficult to read.
Next, a more detailed description will be given of the relationship of the problems peculiar to the low resolution and the ink properties. The image recorded by the ink-jet printer is represented by dots which are arranged in a matrix arrangement in the main scanning direction of the head and in a transport direction (sub scanning direction) of the recording medium which is perpendicular to the main scanning direction.
When the dot image of the characters is recorded, the quality of the characters greater differ depending on the resolution of the image which is recorded. For example, when the character having the same size is recorded at 300 dpi and at 600 dpi, the number of dots forming the character for the 600 dpi becomes approximately four times that for the 300 dpi. Hence, a more detailed representation can be made at 600 dpi, and the quality of the character is naturally better at 600 dpi than at 300 dpi. Particularly at an oblique line portion of the character, the number of dots increases or decreases in steps depending of the resolution, and a jaggy portion becomes more conspicuous at 300 dpi than at 600 dpi.
Accordingly, the jaggy portion which is not conspicuous when the ink used introduces feathering, becomes conspicuous when the ink used introduces no feathering or only introduces slight feathering. The j aging portion becomes particularly conspicuous at the time of the low-resolution recording and deteriorates the quality of the characters.
For example, a Japanese Patent No.2886192 proposes a method of reducing the jaggy portion to improve the quality of the output image. According to this proposed method, a bit pattern of a sampling window within a bit-map image of the character is compared with a predetermined bit pattern, and a center pixel within the sampling window is corrected to a small dot if the compared bit patterns match.
A similar method is proposed in a Japanese Patent No.3029533. According to this proposed method, a contour portion of the image is discriminated from black dot data, and the size of recording dots other than edge dots and black dots is reduced.
Of the above described recording methods proposed in the Japanese Laid-Open Patent Applications No.6-171072 and No.2000-355159 in order to solve the problems introduced when carrying out the recording on the plain paper using the pigment-based ink, the recording method proposed in the Japanese Laid-Open Patent Application No.6-171072 can improve the recording tone and reduce the feathering, because a contact angle of the ink used is 70 or greater and extremely high with respect to the sized recording paper such as plain paper.
However, when recording at 100% duty with respect to the recording paper, the solid adhering amount per unit area needs to be on the order of approximately several tens of ng/m2, and problems are introduced from the point of view of the ink fixing characteristic (drying characteristic). Particularly in a case where a high-speed recording is carried out by stacking a plurality of recording paper, the ink transfer between the successive recording paper causes ink smear on the recording paper. Thus, the recording method proposed in the Japanese Laid-Open Patent Application No.6-171072 is not suited for the high-speed recording.
In addition, depending on the kind of recording paper, the large contact angle of the ink generates a white line of the paper texture at the solidly-recorded portion, the character portion and the like when the recording is carried out at 100% duty. Furthermore, at the boundary portion of the colors, the color bleeding is easily generated in the ink drop state between the adjacent recorded dots due to the large contact angle of the ink.
On the other hand, the recording method proposed in the Japanese Laid-Open Patent Application No.2000-355159 uses the penetrant, and the picture quality is improved from the point of view of the ink drying characteristic (fixing characteristic). Hence, even when a high-speed recording is carried out by stacking a plurality of recording paper, the ink transfer between the successive recording paper does not occur and no ink smear is formed on the recording paper. Thus, the recording method proposed in the Japanese Laid-Open Patent Application No.2000-355159 is suited for the high-speed recording.
However, since the penetrant is used in the ink composition, a feathering phenomenon which is similar to that generated when using the dye-based ink is generated when the recording is made on plain paper. Particularly when the recording is made on the plain paper, the ink penetrates the plain paper also in a direction along a depth (thickness) of the plain paper. As a result, the penetrating ink becomes visible on the other side of the plain paper, thereby making the recording unsuited for application to the duplex recording.
Therefore, the recording methods proposed in the Japanese Laid-Open Patent Applications No.6-171072 and No.2000-355159 cannot realize a sufficiently satisfactory picture quality when carrying out the high-speed recording with respect to the plain paper using the pigment-based ink.
On the other hand, the methods proposed in the Japanese Patents No.2886192 and No.3029533 for correcting the jaggy portion cannot be applied as they are to the ink-jet printer for the purposes of improving the picture quality when recording the image having the low resolution using the ink which generates little feathering.
In other words, the above described methods for correcting the jaggy portion employ toners having a grain diameter of 10 μm or less, as in the case of LED printers and laser printers. Hence, the dots recorded on the plain paper virtually does not spread, and the above described methods for correcting the jaggy portion are effective only with respect to image forming apparatuses in which the dots having the specified small size are obtainable. In addition, the above described methods for correcting the jaggy portion can be employed in the laser printers because the dots having the specified size can be formed at optimum positions by finely varying the positions (timings) and lengths of the laser emissions.
But in the case of the ink-jet printer, the spreading of the ink dot is large compared to that of the toner dot in the laser printer, and the time required to form the dots is large compared to the LED printers and the laser printers. Consequently, it is difficult to vary the dot size among a large number of dot sizes by varying the number, length and the like of driving pulses during the driving period, and it is only possible to vary the dot size among several sizes at the most. Furthermore, the dot can only be formed at an approximately predetermined position within one pixel due to similar reasons, and it is difficult to freely vary the position of the dot within one pixel as is possible in the case of the LED printers and laser printers.
Therefore, the methods proposed in the Japanese Patents No.2886192 and No.3029533 for correcting the jaggy portion cannot be applied as they are to the ink-jet printer for the purposes of improving the picture quality.
Moreover, there is a conventional smoothing technique called anti-aliasing. However, the anti-aliasing varies the dots in an extremely large number of gradation levels at the contour portion. For this reason, a highly accurate smoothing can be realized by the anti-aliasing, but the required processes are extremely complex and time-consuming. Consequently, the anti-aliasing is unsuited for application to the ink-jet printer which is required to produce a high throughput.
In addition, when the recording is made by varying the dot size in the ink-jet printer, there is a problem in that the dot position on the recording paper becomes different depending on the dot size. The ink-jet printer applies pressure to the ink within the ink chamber by the pressure generating means, and the ink is ejected from the nozzle by the pressure applied to the ink within the ink chamber. The pressure generating means is formed by the heating resistor which generates the air bubbles in the case of the thermal ink-jet printer, and is formed by the piezoelectric element (electromechanical conversion element) which deforms the ink chamber i n the case of the piezoelectric ink-jet printer.
Generally, the energy supplied to the pressure generating means is varied in order to vary the dot diameter in such ink-jet printers. More particularly, the magnitude of the driving voltage applied to the pressure generating means is varied or, the pulse width or the number of pulses of the driving pulses applied to the pressure generating means is varied.
The method of varying the driving voltage requires a number of signal lines corresponding to a plurality of driving voltages, and a number of switching means, corresponding to the plurality of driving voltages, for switching the plurality of driving voltages with respect to each channel. As a result, a driving element (driver IC) becomes large. On the other hand, the method of varying the pulse width or the number of pulses of the driving pulses can vary the pulse width or the number of pulses by controlling a switching timing of a switching means, and only a single switching means is required with respect to each channel. Hence, particularly in the case of the piezoelectric ink-jet printer, the pulse width modulation method or the pulse number modulation method is employed.
However, when forming ink drops having different amounts of ink, that is, when forming different dot diameters, according to the pulse width modulation method or the pulse number modulation method, the length of the driving pulses become different. As a result, even though the timings at which the meniscus is formed in response to the driving pulse is the same for the ink drops having the different amounts of ink, the timings at which the ink drops are ejected in response to the ceasing of the driving pulses become different. Consequently, the timings at-which the ink drops having the different amounts of ink reach the recording paper become different, and the dot position (ink drop hitting positions) on the recording paper become different depending on the dot size. Therefore, even when an attempt is made to improve the picture quality by correcting the contour portion by the small dots, the small dots are not formed at the desired positions on the recording paper, and there is a possibility of deteriorating the picture quality rather than improving the picture quality.